1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Discussion of the Background
Electrophotographic images are formed, for example, through processes of charging, irradiation, development and transfer conducted around a photoreceptor functioning as a latent image bearing member in an electrophotographic image forming apparatus.
In such image formation, corona products produced in the charging process and un-transferred residual toner may remain on the image bearing member after the transfer process. Therefore, the image bearing member is subject to a cleaning process after the transfer process to remove such corona products and residual toner.
Cleaning blade systems using a rubber blade are known and typically used as the cleaning system for use in such a cleaning process since the rubber blade is inexpensive, simple and of good cleaning property.
However, the rubber blade is pressed against the surface of the image bearing member to remove the residuals thereon, which causes substantial friction stress between the surface of the image bearing member and the cleaning rubber blade. Therefore, the rubber blade and the surface layer of the image bearing member, particularly in the case of an organic photoconductor, are abraded, which shortens the actual working life of the rubber blade and the organic photoconductor.
In addition, toner having a small particle diameter has come to be widely used for image formation to respond to the demand for improvement in image quality. In an image forming apparatus using a toner having a small particle diameter, the proportion of un-transferred residual toner that slips through the cleaning blade significantly increases, particularly when the dimensional accuracy and assembly accuracy of the cleaning blade are low, and/or when the cleaning blade partially vibrates, thereby degrading the image quality. Therefore, improvement of the cleaning property by reducing the deterioration of members due to abrasion is required to make the actual working life of an organic photoconductor longer, and output quality images for an extended period of time.
Friction between the blade and the photoconductor is typically reduced by supplying and applying a lubricant to the surface of the organic photoconductor followed by even application of the supplied lubricant to the surface with the cleaning blade or brush to form a lubricant film. Refer to unexamined published Japanese patent application publication No. (hereinafter referred to as JP-A) 2000-162881-A, etc.
Although successful, with this approach it is necessary to determine in advance the precise amount of lubricant to be applied. An excessively small amount of the lubricant leaves such problems unsolved that the organic photoconductor is not protected from abrasion or damage, or the blade is still easily degraded. By contrast, when an excessively large amount of the lubricant is supplied, excess lubricant accumulates on the surface of the organic photoconductor, which leads to image flow, or mixes with a development agent, resulting in degradation of the performance of the development agent
On the other hand, in a typical method of improving the cleaning property, a lubricant is externally added to the toner for use in development and supplied to the latent image bearing member only when developing an image with the toner.
JP 2002-229241-A describes a method in which friction between the latent electrostatic image and the cleaning blade is reduced by the supply of a lubricant and the cleaning ability for the residual toner is secured. However, as described in JP 2002-229241-A, when a lubricant is externally added to a toner, the lubricant is applied only to the toner image formed portion on the surface of a latent image bearing member. When a large quantity of data of, for example, an estimate, or a project protocol having an image portion clearly distinct from a non-image portion in a single image, or data having large image density differences depending on which portion of one image are printed on recording media such as sheets, the lubricant is not supplied to the portion where no toner image is formed on the latent image bearing member. That is, lubricant application is localized.
Consequently, the latent image bearing member tends to be locally abraded and the cleaning blade easily vibrates at the border between the portion where the lubricant is applied and the portion where the lubricant is not applied. In addition, this leads to problems such as poor cleaning performance and squeaky noise disturbance.
Furthermore, the amount of the lubricant, which is externally added to toner (for use in a development agent), applied to a latent image bearing member varies depending on the image density. As a result, the amount of lubricant applied decreases with regard to a portion having a thin image density so that abrasion or damage on the latent image bearing member or deterioration of the cleaning blade is not sufficiently prevented. When the image density is thick or the proportion of the lubricant externally added to toner is too high, the amount of the lubricant applied to the latent image bearing member easily increases to a degree that excessive lubricant thereon causes image blur due to image flow on the end portion of the image portion, or lubricant transfers to the charging roller, resulting in variation of the resistance of the charging roller, which leads to a problem of insufficient charging, depending on the image formation conditions. Therefore, the lubricant applied to a latent image bearing member is required to keep an optimal amount.
As the method of using toner to which a lubricant is externally added as described in JP 2002-229241-A, for example, JP 2003-241570-A describes a method in which a solid toner image is formed on the entire surface of a latent image bearing member before image formation starts so as to supply lubricant.
Although lubricant is supplied to the entire surface of a latent image bearing member by using the method described in JP 2003-241570-A, a great amount of the development agent is used, thereby increasing the amount of toner waste, which is a heavy burden on the environment.
In addition, outputting a solid image is not limited to the timing before image formation starts. Such a solid image is periodically output over time in order to prevent local uneven abrasion of the latent image bearing member.
As described above, a great amount of toner waste is typically discharged in exchange for prevention of uneven local abrasion of a latent image bearing member.
In addition, abrasion and image blur can be caused not just by too much lubricant or too little, but also by the interaction between the lubricant and the latent image bearing member onto which the lubricant is applied. For example, a lubricant such as metal soap covers all over the surface of a latent image bearing member, meaning that the lubricant has a function of protecting the surface from the discharging energy of a charging device. However, protecting the surface of a latent image bearing member from the discharging energy means that the lubricant absorbs the energy, thereby degrading the lubricant film.
JP 2008-139804-A attempts to solve this problem, and describes a method in which a lubricant functions as the protection film by regulating the application amount of the lubricant while reducing unwanted side effects. However, when degraded lubricant is left on the surface of a latent image bearing member under high-temperature, high-humidity conditions, significant image blur tends to occur particularly immediately below the charging device. This image blur is particularly noticeable when a latent image bearing member having a cross-linked surface structured by cross-linking a radical polymerizable compound is used.
Although the mechanism of this phenomenon is not clear, one possible reason is that degraded lubricant, moisture in the atmosphere, and corona products produced by a charging device bond together, thereby reducing the resistance of the surface, resulting in image flow of a latent electrostatic image.
In addition, another possible reason why this phenomenon occurs particularly to a latent image bearing member having a cross-linked surface structured by cross-linking a radical polymerizable compound is that degraded lubricant is hardly removed from the surface, and so is hardly replaced with fresh lubricant.
It is possible to increase the amount of the lubricant supplied. However, fresh lubricant is just applied onto the degraded lubricant attached to the surface of the latent image bearing member. Therefore, increasing the amount of lubricant does not contribute to replacement of the degraded lubricant and is actually not effective to solve the image flow problem.
On the other hand, when the amount of lubricant applied to the surface of a latent image bearing member is reduced, the lubricant on the surface is slightly easier to remove, although at the cost of increased abrasion of the surface of the latent image bearing member.
When the cross-linked surface layer of a latent image bearing member has a high content ratio of oxygen atom, the oxygen easily becomes an active spot of a radical, etc. due to discharging in the electrophotographic process. Therefore, deterioration of the surface due to discharging is aggravated, or corona products such as ozone and nitrogen oxides are easily attached to the surface, thereby degrading the cleaning property or image quality.
To deal with this issue, for example, JP 2007-156081-A describes a method of abrading a portion of the surface having a high content ratio of oxygen atoms or curing the surface layer in an inert gas atmosphere. However, in an image forming apparatus having a lubricant application mechanism, which has come to be widely used in recent years, not only the surface of a latent image bearing member but also the lubricant applied thereto are possibly degraded by discharging.
In addition, different from a latent image bearing member vulnerable to abrasion, the degraded lubricant is hardly removed together with the surface of a latent image bearing member having a cross-liked surface layer because of its extremely strong abrasion resistance. Therefore, unless the lubricant is smoothly replaced with fresh lubricant, image blur tends to occur in high-temperature, high-humidity environments.
As described above, when a highly durable latent image bearing member because of its cross-linked surface layer formed by cross-linking a radical polymerizable compound is used in an image forming apparatus having a lubricant applicator, the image blur in under high-temperature, high-humidity conditions and the abrasion of the latent image bearing member are inversely related.
Furthermore, when an image forming apparatus employs a counter-blade cleaning system having a urethane rubber blade, in which the urethane rubber blade is brought into contact with the latent image bearing member against the rotation direction thereof to remove un-transferred residual toner, a lubrication material such as silicone oil is added to the surface of the latent image bearing member to reduce the initial torque. Such a lubrication material tends to ooze to the surface, which blocks cross-linking of a radical polymerizable compound. Therefore, replacement of the lubricant applied to the surface on which silicone oil is present in large amounts tends to be hindered, which leads to frequent occurrence of image blur.